The present invention relates to an arrangement for sealing a reciprocating rod relative to a housing part surrounding the rod in slightly spaced relationship, in particular for use in shock absorbers, comprising a sealing ring fitted in a groove in the housing part, the sealing ring consisting of a tough-elastic plastic material, having a diameter smaller than that of the groove and carrying on its inside, near its high-pressure end, an edge forming the beginning of a conical surface opening towards the low-pressure end and forming with the circumferential surface of the rod a wedge-shaped gap of a small wedge angle, and comprising further a stressing ring of a rubber-elastic material resting against the base of the groove and being loaded between a flank of the groove and the neighboring end face of the sealing ring, for retaining the sealing ring in contact with the opposite flank of the groove.
Such an arrangement has been known already from DE 27 05 081 A1. In the case of this known seal, the stressing ring is arranged at the low-pressure end so that the fluid to be sealed between the circumferential face of the sealing ring and the base of the groove fills the existing gap. It is ensured in this manner that the force by which the edge of the sealing ring is pressed against the rod is substantially proportional to the pressure of the fluid to be sealed off. The stressing ring is arranged in such a manner that it exerts simultaneously a radially-directed pressing force upon the sealing ring and that the stressing forces are directed upon the sealing edge.
An arrangement in which the stressing ring is located on the high-pressure side is used also in the double-stripper arrangement known from DE 36 03 669 A1.
It has been found that in seal arrangements of this type friction increases in the course of operating time. This is due to the fact that the contact surface formed by the edge gets larger and tends to flatten. In the case of operating pressures varying within broad limits, which is the case in particular with shock absorbers, it may in addition happen that the pressing force, having been adjusted correctly for low operating pressures, rises considerably in the case of high operating pressures, which may also lead to undesirably high friction values.
Now, it is the object of the present invention to provide a seal arrangement where the friction value rises as little as possible as the differential pressure acting upon the sealing arrangement increases, and where the friction value increases also as little as possible in the course of operating time. This object is achieved for an arrangement of the type described at the outset by the fact that the edge forms the starting point also for a conical face opening towards the high-pressure end and that the stressing ring is arranged at the high-pressure end of the sealing ring.
The arrangement of a conical surface opening toward the high-pressure end, though it may be very short, imparts to the edge improved stability so that it shows a reduced tendency to flatten. In addition, the arrangement of the stressing ring at the high-pressure end of the sealing ring prevents the sealing ring from being loaded on its outside, by the fluid to be sealed off, over its full length. Rather, it is now possible, by giving the contact surface an appropriate configuration, to meter exactly the effect of the fluid pressure on the pressing force prevailing at the edge and to make the latter largely independent of the fluid pressure.
The seal arrangement according to the invention permits the most varied solutions regarding the design and arrangement of the stressing ring, as well as regarding the contact face on the sealing ring for the stressing ring. For example, the groove provided in the housing part may have a section of larger diameter at its high-pressure end, and the stressing ring may in part engage the annular recess formed by this section of larger diameter. Such an arrangement may be convenient in particular when the stressing ring is in contact with a plane end face of the sealing ring.
According to a preferred embodiment, however, the arrangement is such that the stressing ring loads the sealing ring radially over a section of its outer surface extending from the high-pressure end of the sealing ring to about that plane which is defined by the edge of the sealing ring being in contact with the rod. As the fluid pressure acting on that section of the inner face of the sealing ring which extends from the sealing edge to the high-pressure end of the sealing ring has the tendency to lift the sealing ring off the rod, this pressure is compensated by the pressure acting on the sealing ring from the outside, via the stressing ring. Depending on whether the outer face being loaded radially by the stressing ring ends before or behind the plane defined by the edge, a state of undercompensation or overcompensation will occur. A slight overcompensation may help ensure that a safe sealing effect is always obtained even if the edge pressure resulting from a certain oversize of the sealing ring should decrease, or even disappear completely in the course of time, while on the other hand the pressing force resulting from the fluid pressure will not rise excessively in the presence of high pressures. However, such compensation can already be achieved solely by the fact that the outer surface of the sealing ring is stepped in such a manner that the sealing ring exhibits a section of reduced diameter extending from the high-pressure end of the sealing ring approximately to the plane defined by the edge of the sealing ring which is in contact with the rod. In this case, it is of no importance for the compensation of the fluid pressure whether or not the stressing ring is in contact with the section of reduced diameter of the sealing ring. If the stressing ring is in contact with the section of reduced diameter of the sealing ring, then it would be particularly preferable to give the stressing ring the design of a square ring. The spacing between the end of the radially loaded section of the outer surface and the plane defined by the edge should not exceed 1 mm.
According to a particularly preferred embodiment of the invention, a radially outer section of the end face of the sealing ring is formed by a conical surface intersecting the circumferential face of the sealing ring in the area of the plane defined by the edge which is in contact with the rod. One obtains in this case an annular space of substantially triangular cross-section in which the correspondingly sized stressing ring, being preferably an O ring, is retained in compressed condition so that it rests against the outer conical surface of the sealing ring by the force resulting from the initial pressure. This force comprises on the one hand an axial component by which the sealing is pressed against the opposite flank of the groove, and on the other hand a radial component leading to a constant edge pressure. It is possible in this manner to achieve a particularly good sealing behavior of the seal, with a very simple structure. Conveniently, the angle formed between the conical face forming part of the end face and the plane defined by the edge should be equal in this case to 20.degree. to 40.degree..
It is important for the function of the arrangement according to the invention that the sealing ring be given sufficient bending resistance. A body of high bending resistance is obtained when the conical surface on the high-pressure end extends along the inner face of the sealing ring up to the high pressure end of the sealing ring so that it intersects the conical surface which forms part of the end face and which also forms the contact surface for the stressing ring. With respect to this conical surface it is also advantageous if the angle included between this surface and the plane defined by the edge is equal to 20.degree. to 40.degree..
Furthermore, the conical surface on the low-pressure side, on the inside of the sealing ring, may form with the rod surface an angle of 5.degree. to 15.degree..